1. Field of the Invention
The present invention relates to an adaptive spread spectrum frequency-hopped multilevel frequency shift keyed (FH-MFSK) transmitter and receiver and, more particularly, to an adaptive spread spectrum FH-MFSK transmitter and receiver which permit the control of an associated user's performance by the selective simultaneous transmission of one or more encoded frequencies per time slot for the transmitted sequence such that greater system flexibility is provided for choosing operating parameters and/or that the performance of users with the worst reception conditions can be improved but at the expense of those users with better reception conditions resulting in an overall increased system efficiency.
2. Description of the Prior Art
With limited frequency spectrum available, efficient mobile radiotelephony service to a larger number of users requires that many users be able to share the same channel without interfering with each other. The cellular approach, where the area is divided into cells and each user communicates with a base station in his cell, is a widely recognized solution to this problem. An example of a mobile radio system using a hexagonal cell configuration and centrally located base stations transmitting and receiving frequency division multiplexed signals is disclosed in the article "The Advanced Mobile Phone Service" by N. Ehrick in IEEE Communications Magazine, Vol. 17, No. 2, Mar. 1979 at pp. 9-15. Other techniques such as space diversity, corner base stations and spread spectrum have been proposed with the goal of increasing the spectrum efficiency in the multipath fading conditions which characterize the mobile radio environment. In this regard see, for example, the articles "A Spread Spectrum Technique for High-Capacity Mobile Communications" by G. R. Cooper et al. in IEEE Transactions on Vehicular Technology, Vol. VT-27, No. 4, Nov. 1978 at pp. 264-275; "A Processing Satellite Transponder for Multiple Access by Low-Rate Mobile Users" by A. J. Viterbi in the Conference Record of the Fourth International Conference on Digital Satellite Communications at Montreal, Canada on Oct. 23-25, 1978 at pp. 166-174; and "Frequency Hopped Multilevel FSK for Mobile Radio" by D. J. Goodman et al. in the Conference Record of the IEEE 1980 International Zurich Seminar on Digital Communications, Mar. 4-6, 1980 at pp. A5.1-A5.6.
What is common to all those schemes is that the reception conditions vary throughout the cell and are usually worst near the boundary (or corner) where the desired signal is the weakest and the interference from other cells is the strongest. Thus most users get "too much" energy in order to accommodate those in the worst conditions. This excessive energy which is transmitted increases the interference and reduces the efficiency of the system. Hence it is desirable to have an adaptive system which will try to equalize the reception quality for all users by making adjustments in transmission. In a single frequency per channel system the former equalization could be done, in principle, with some form of power control.
The problem remaining in the prior art is to provide an adaptive system for the spread spectrum FH-MFSK radio system. Another characteristic of the spread spectrum FH-MFSK system is that the total available bandwidth and the user's data rate constrain the choice of the operating parameters (the number of frequencies and the number of hops per codeword), and any deviation from the optimum choice sharply reduces the system efficiency. Thus the problem remaining in the prior art is to provide better flexibility in system design.